An association between hormone receptors in primary breast cancer and obesity determined prior to mastectomy was investigated in 128 Japanese women. The following criteria for obesity were used: (1) weight greater than or equal to 60 kg (132 lbs), (2) weight kg/height cm-105 greater than or equal to 1.3, (3) weight lbs/height in greater than or equal to 2.30, (4) body surface area greater than or equal to 1.56 m2. In view of the 4 criteria, tumor estrogen receptor (ER) values greater than or equal to 4 fmol/mg were observed in obese patients more often than in nonobese patients, though the difference was not statistically significant. The same tendency was observed in the postmenopausal subgroup, 62 patients, especially in the 36 patients more than 5 years beyond menopause. However, there was still no statistical difference between obese and nonobese patients because the number of subjects was small. The same tendency was observed in the case of progesterone receptor (PgR) (greater than or equal to 6 fmol/mg) as observed in the case of ER.